(1) Field of the Invention
The present invention relates to an apparatus for extracting digital audio information from data which contains digital audio information and is transmitted by way of a radio transmission path, and for decoding the extracted digital audio information and reproducing an audio signal. More particularly, the present invention relates to a digital audio reproducing apparatus suitable for use in a mobile receiver for receiving digital broadcasting programing broadcast by a satellite.
(2) Description of Related Art
Recently, in parallel with putting digital satellite broadcasting into practice, there have been developed and proposed various image data compression system, audio data compression systems and the like. Also, there are discussed systems of various manner for receiving digital satellite broadcasting by a receiver carried in a mobile unit.
When satellite broadcasting is to be received, in general, it is necessary to prepare a parabolic antenna for receiving the radio wave of which frequency is allocated by an authority. Therefore, it was unrealistic for a user of the mobile unit to prepare such an antenna to receive the satellite broadcasting. However, since S-band (2.6 GHz band) frequency (which is particularly unsusceptible against rain), is allocated for mobile units to receive the satellite broadcasting, it is realistic for the mobile user who lacked receiving means to now receive sought-after satellite broadcasting.
When data containing digital audio information is transmitted from a geostationary satellite to a portable receiving terminal or a terminal carried in a vehicle on the ground in a broadcasting situation with mobile units as targets, the data containing digital audio information is sometimes dropped midway of the radio wave transmission. This occurs when the mobile unit passes through the shadow place of the broadcast radio wave such as a crowded group of buildings, trees, a bridges under a tunnel and so on, such that the broadcasting radio wave is prevented from being transmitted by these obstacles, resulting in a broadcast break on the receiving side. In the field of broadcasting technology, it is substantially impossible to send data repeatedly from the broadcasting station to each of the plural receivers upon each repeating broadcasting request. Therefore, it is essential to keep a reproduction of the broadcasting data even at breaks in the radio wave reception.
In a receiving environment in which the mobile unit receives radio waves as described in the above manner, when a mobile unit on the ground fails to receive transmitted data from the geostationary satellite in a normal fashion, a disturbance is caused in a picture or a break in audibility. Although it is relatively easy to reduce the annoyance of a video program viewer from the disturbance in picture reproduction, it is quite difficult to reduce the annoyance coming from breaks in the sound with the mere countermeasure of muting or the like. Particularly, when audio broadcasting is provided for mobile unit users and the driver of a vehicle is listening to a broadcast of sound intensively, any sound breaks are truly annoying. Thus, there is a great need to improve audibility in such conditions.
The following novel measures have been taken to resolve the above-mentioned challenges.
{circle around (1)} When the radio wave transmission is interrupted due to any obstacle, novelty the receiving terminal stops broadcast reproduction.
{circle around (2)} If the xe2x80x9cshadow areaxe2x80x9d of the broadcasting satellite is caused in a wide area, a gap filler (retransmission equipment) or similar equipment is installed to reduce the radio wave shadow area. Alternatively, a plural number of broadcasting satellites remote from one another are utilized for transmitting radio waves to novelly resolve the above challenges.
{circle around (3)} Error correction functions, for example, one using an interleave, intraframe coding, interframe coding and so on are driven to restore lost data, and depth of the interleave and code length of the error correction code are optimized to cope with the obstacle of the radio wave transmission gap.
{circle around (4)} Radio wave transmission is carried out by using a time diversity system in which the same transmission data is transmitted with a time lag.
However, a break in transmission interrupts as described in item {circle around (1)} above. Because of this break in transmission the annoyance to a listener is unacceptable. Particularly when an audio broadcast is provided for mobile unit users, such interruption in the radio wave reception draws attention to the user with a broken signal so there is a definite need to avoid such interruptions. The avoidance of such interruptions may be solved as disclosed herein by the invention.
The countermeasure gap filler introduced in item {circle around (2)} is effective because radio waves are supplied from the gap fillers to the xe2x80x9cshadow areaxe2x80x9d of the satellite radio wave which can extend beyond the xe2x80x9cshadow areaxe2x80x9d gap in transmission caused by buildings. However, there are numerous xe2x80x9cshadow areasxe2x80x9d across the country such as the xe2x80x9cshadow areasxe2x80x9d of groups of small buildings, trees, a large-sized vehicle approaching from an opposite side of a road and the like. Therefore, it is unrealistic from an economic standpoint to install gap fillers to eliminate all possible xe2x80x9cshadow areas.xe2x80x9d Numerous areas are left in which it is incapable of receiving a radio wave. Further, even if a reasonable number of gap fillers are installed, the economic constraints will result in the xe2x80x9cshadow areasxe2x80x9d being made too small, the radio wave receiver eventually moving to another area in which it becomes capable of receiving the radio wave again. However, if the receiver is carried in a mobile unit such as a motor vehicle, when the vehicle passes through an area in which the radio wave is not supplied it is inevitable to have a point in which the radio waves are not received. Furthermore, if another mobile unit is as an obstacle preventing radio waves from being transmitted, the temporary lack of reception of the radio wave is brought about.
On the other hand, if a plurality of broadcasting satellites are available for transmitting radio waves, it is possible to reduce the areas in which the radio waves are not received. However, it is difficult to achieve a reasonable effect due to the economic cost of providing satellite coverage. For example, if a relatively inexpensive geostationary satellite for use as a BS broadcasting is utilized to serve also as the audio signal broadcasting means, the greater the area in which the radio wave reception is attempted apart from the equator, the smaller angle of elevation directing the north or south orientation now becomes available for receiving radio waves. Further, if a LEO (Low Ear Orbit) satellite is employed for supplying radio waves, for example, more than totally eight satellites may be utilized; with at least four satellites on each of the two orbits intersecting with each other at right angle, at a great amount of installation cost.
If the scheme of interleave or error correction introduced in the item {circle around (3)} is employed, the bit error cannot be eliminated completely. For example, if a coded data compression system such as an MPEG system is employed, decoding is carried out at the unit of the frame. Therefore, there can exist a case in which a frame is deleted in spite of the fact that the frame contains only one bit error. Yet, much redundant data will be generated adding for error correction, resulting in the deterioration of radio wave utilization efficiency.
Further, if the time diversity system described in item {circle around (4)} is employed, another carrier wave will be prepared, leading to deterioration in the channel utilizing efficiency.
The present invention in view of the above aspects, with an object of the present invention, for example, to provide a digital audio reproducing apparatus of a simplicity which provides a solution of the challenges of when a radio wave cannot be received temporarily, such as when the apparatus is utilized with a mobile terminal in a mobile satellite broadcasting system at an S-band frequency. In such an example, reproduction is carried out with satisfactory audibility even though the reproduced audio signal suffers from interruption due to the break of the broadcasting radio wave. Another object of the present invention is to provide a digital audio reproducing apparatus of simplicity in which provides continued broadcasting reproduction with no audibility challenges without the difficulty of preparing another carrier wave and increasing the number of relaying stations, without deteriorating the efficiency of radio wave utilization.
Accordingly, the present invention, for example, provides a digital audio reproducing apparatus which includes a receiving means for receiving modulated data containing coded digital audio information sent in a unit of frame, demodulating means for demodulating the modulated data received by the receiving means, audio decoding means for decoding in a unit of frame digital audio information contained in the modulated data demodulated by the demodulating means, and audibility correcting means for carrying out audibility correction. The audibility correcting means uses at least one sample of digital audio information selected from out of forward (forward indicates signal elements received or processed immediately ahead of the element being examined) digital audio information that has been sent before failing digital audio information accommodated in a frame fails to be decoded and has been successfully decoded by the audio decoding means and backward (backward indicates subsequent signal elements that were transmitted after the element preceding) digital audio information that has been sent after the failing digital audio information and has been successfully decoded by the audio decoding means, when the audio decoding means fails to decode the digital audio information. According to the above arrangement, it is possible to carry out correction of audibility from a practical standpoint without increasing the number of gap fillers or relaying stations, and it is even possible to continue broadcasting reproduction with the broadcasting radio wave interrupted due to obstacles. Therefore, investment cost can be reduced.
According to the present invention, the audibility correcting means may be arranged to carry out the audibility correction by using only the forward digital audio information. Alternatively, the audibility correcting means may be arranged to carry out the audibility correction by using only the backward digital audio information. Further, the audibility correcting means may be arranged to carry out the audibility correction by using both the forward digital audio information and the backward digital audio information.
According to the above arrangement, it becomes possible to carry out broadcasting reproduction via a simple process without a large investment on gap fillers. Further, as a satellite station need not transmit redundant data, the radio wave frequency band can be more effectively utilized.
According to the present invention, the audibility correcting means may comprise a first smoothing processing means for smoothing the boundary between corrected data that has been subjected to the audibility correction by using the forward digital audio information or the backward digital audio information and non-corrected data that has not been subjected to the audibility correction.
According to the above arrangement, natural sound with no noise from an audibility standpoint can be made available by a novel simple apparatus at a low cost.
According to the present invention, the audibility correcting means may be arranged to carry out an averaging process via an inclined weight distribution on the forward digital audio information and the backward digital audio information to create corrected data from subjection to the audibility correction.
According to the above arrangement, interpolation can be carried out regardless of the correlation in audio data between frames which are placed closely to each other. Therefore, it is possible to provide an apparatus with a simple arrangement in which even with the mobile receiving terminal traveling into a xe2x80x9cshadow areaxe2x80x9d of the radio wave, satisfactory sound audibility is achieved and continuously available without any interruption. Accordingly, it is possible to reduce the cost of providing expensive equipment such as a gap filler or the like, with significant cost reduction via the disclosed system.
Accordingly, the present invention provides a digital audio reproducing apparatus including receiving means for receiving modulated data containing coded digital audio information sent in a unit of frame, demodulating means for demodulating the modulated data received by the receiving means, audio decoding means for decoding in a unit of frame digital audio information contained in the modulated data demodulated by the demodulating means, and audibility correcting means for carrying out audibility correction by deleting failing digital audio information accommodated in a frame which has failed to be decoded, when the audio decoding means fails to decode the digital audio information.
According to the above arrangement, it is possible to carry out a satisfactory audibility correction pragmatically without increasing a number of gap fillers or relaying stations, and it becomes possible to continue broadcasting reproduction even if the broadcasting radio wave is interrupted due to obstacles. Therefore, investment cost can be reduced.
According to the present invention, the audibility correcting means, for example, is arranged such that when the audibility correcting means deletes the failing digital audio information and places pieces of the digital audio information neighboring to the failing digital audio information to achieve audibility correction, the audibility correcting means selects a position where the pieces of the digital audio information neighboring the failing digital audio information have the audio signal levels most coincident to each other.
Further the present invention provides that the audibility correcting means can be arranged where the audibility correcting means deletes the failing digital audio information and thereby places pieces of the digital audio information neighboring the failing digital audio information proximate each other achieving audibility correction, as the audibility correcting means selects a position where the pieces of the digital audio information neighboring the failing digital audio information have the audio signal levels and the audio signal slopes most coincident to each other.
In addition, the audibility correcting means can include a second smoothing processing means for smoothing the boundary caused when the audibility correcting means deletes the failing digital audio information and places the pieces of the digital audio information neighboring the failing digital audio information close to each other, in another embodiment.
According to the above arrangement, the audio signals are connected to each other at the most appropriate position from an audibility standpoint. Therefore, excellent audio reproduction is achieved, with natural sound with no noise from an audibility standpoint achieved.
Further, the audibility correcting means can include means for time-adjusting data insertion inserting time adjusting data for correcting a time lag caused when the audibility correcting means deletes the failing digital audio information. The time adjusting data inserting means can be arranged to insert 0-level data (0-level adjustment data indicates an initial time-measuring sample value set at zero time for achieving time adjustment) or minute level data (minute level adjustment data indicates a value sampled at minute durations for achieving time adjustment) into the digital audio data at a position with a small audio signal level. Further, the means for time adjusting data insertion can be arranged to create the time adjusting data from pieces of the digital audio information neighboring the failing digital audio information. Further, the time adjusting data inserting means can be arranged to insert the time adjusting data into the digital audio data at a position having an absolute value of the volume change amount of the audio signal larger than a first set value.
According to the above arrangement, even if a number of frames are placed close to each other, the data stream can achieve real tune broadcasting because time adjustment is carried out. Furthermore, even if the time adjusting data to be inserted contains or causes noise, the disclosed invention allows the magnitude of the noise to become relatively small with respect to the signal level near the position at which the time adjusting data is inserted. Therefore, the noise is masked by the high signal level position, hence the noise is barely discernible.
According to the present invention, the means for time adjusting data insertion can be arranged to search a predetermined range of the digital audio information after the position where the failing digital audio information is deleted, for a position at which an absolute value of the volume change amount of the audio signal is smaller than a second set value, and to insert the time adjusting data thereat. Further, the second set value is arranged to be variable so that it has a positive correlation with a mean volume value within a predetermined time range or a predetermined number of frames or a volume value obtained by an averaging process with an inclined weight distribution carried out such that a preceding portion remote (i.e. a preceding portion distant) from the current position is applied with a smaller weighting coefficient.
Accordingly, broadcasting data of an accurate time is achieved without any audibility challenges.
According to the present invention, the means for time adjusting data insertion is arranged to search a predetermined range of the digital audio information after the position where the failing digital audio information was deleted, for a position where the volume of the audio signal becomes smallest, and inserts the time adjusting data thereat.
According to the above embodiment, even if a number of frames are placed close to each other, the data stream can follow the real time property of the broadcasting because of the time adjustment achieved. Furthermore, even if the tune adjusting data to be inserted contains or causes noise, the volume of the noise itself becomes small and hence not conspicuous. Particularly when the mobile unit is operated on at an ordinary noise level, the noise is almost non-discernible from an audibility standpoint.
Further, according to the present invention provides a digital audio reproducing apparatus with a receiving means for receiving modulated data containing coded digital audio information sent in a frame unit, demodulating means for demodulating the modulated data received by the receiving means, audio decoding means for decoding digital audio information contained in the modulated data which is demodulated by the demodulating means, and an audibility correcting means for carrying out audibility correction. The audibility correcting means allows that when the audio decoding means fails to decode the digital audio information, the audibility correcting means deletes the failing digital audio information accommodated in a frame which has failed to be decoded. The audibility correcting means transforms the forward digital audio information having a time dimension which has been sent before the failing digital audio information and has been successfully decoded by the audio decoding means as well as backward digital audio information having the time dimension that has been sent after the failing digital audio information which has been successfully decoded by the audio decoding means, into a frequency domain. The audibility correcting means creates intermediate frequency domain digital audio information from the forward digital audio information and the backward digital audio information having the frequency dimension after the transformation, applies an inverse transformation on the intermediate frequency domain digital audio information obtaining intermediate digital audio information having a time dimension, weights and places the intermediate digital audio information at the position where the failing digital audio information is deleted or proximate thereof, such that audibility is corrected.
According to the present invention, the audibility correcting means may be arranged to multiply the intermediate digital audio information with a window function, placing the resulting digital audio information at the position where the failing digital audio information is deleted or a vicinity thereof.
According to the above arrangement, it is possible to achieve satisfactory practical audibility correction without increasing a number of gap fillers or relaying stations, and further achieve continued broadcast reproduction even with broadcasting radio wave interruption due to obstacles at reduced cost.
According to the present invention, there is provided a digital audio reproducing apparatus for use in a mobile unit which receives and reproduces modulated coded data containing digital audio information sent in a unit of frame from an accumulating medium or a transmitting medium by way of a satellite, including receiving means for receiving the modulated data, demodulating means for demodulating the modulated data received by the receiving means, audio decoding means for decoding in a unit of frame digital audio information contained in the modulated data demodulated by the demodulating means, and audibility correcting means for audibility correction on failing digital audio information accommodated in a frame which has failed to be decoded, when the audio decoding means fails to decode the digital audio information.
According to the above arrangement, the audibility correction can be easily carried out upon transmitting broadcasting radio waves and investment in mobile satellite broadcasting system reduced.
Accordingly, the present invention provides a digital audio reproducing apparatus for receiving and reproducing modulated data subjected to an interleave operation at least one of the frames neighboring a desired frame constituting broadcast digital data composed of a plurality of frames that is rearranged such that the frame is placed apart from the desired frame by a predetermined time duration, with receiving means for receiving the modulated data, demodulating means for demodulating the modulated data received by the receiving means, audio decoding means for decoding in a unit of frame digital audio information contained in the modulated data demodulated by the demodulating means, and audibility correcting means for carrying out audibility correction the on failing digital audio information accommodated in a frame which has failed to be decoded, when the audio decoding means fails to decode the digital audio information.
According to the above arrangement, if a mobile unit passes an area in which a signal is not supplied and signal reception is incapable in a burst, continuous frame drop can be prevented audibility correction is achieved.